Economical, non-wearing electrical drive device

ABSTRACT

The invention relates to an economical, non-wearing, electrical permanent magnet drive for actively controlling the rotor position in three degrees of freedom. The stator windings produce superimposed fields with different pole numbers in the pole pitches by unsymmetrical magnetomotive force distributions.

DESCRIPTION

[0001] The magnetic bearing technology includes fields of application ofmachine and apparatus construction with extremely high requirements forthe rotary speed range, the working life, the cleanliness and the sealednature of the drive system—i.e. essentially fields of application whichcannot be realized or can only be realized with difficulty usingconventional bearing techniques. Various embodiments, such as forexample high speed milling and grinding spindles, turbo compressors,vacuum pumps or pumps for high purity chemical or medical products arealready equipped with magnetic bearings.

[0002] The machine cross-sections shown in the following figures aresimply by way of example and partly greatly simplified and serveexclusively for the more precise explanation of the principle ofoperation.

[0003] A conventional magnetically journalled machine (FIG. 1) requires,in addition to a machine unit (1), two radial magnetic bearings (2),(3), an axial magnetic bearing (4), two mechanical touch down bearings(5), (6), as well as a total of ten power converter stages (7), (8),(9), (10) for the control of the motor phases and magnetic bearingphases.

[0004] In the literature there are proposals (FIG. 2) for theintegration of machines and radial magnetic bearings into one magneticstator unit. In one stator there are two separate winding systems (11),(12) for torque winding and levitation force winding which are insertedinto slots in multiple layers. The relationship p₁=p₂±2 basicallyapplies in bearingless motors for a largely decoupled levitation forceformation and torque formation between the winding pole numbers, with p₁or p₂ also representing the pole number of the rotor. Both windingsystems of the motor in FIG. 2 are three-phase. The coils arechord-wound and distributed over several slots, whereby an approximatelysinusoidal flux linkage is achieved. The two windings are composed asfollows:

[0005] 4-pole machine winding (11) (outer): phase 1 (13), phase 2 (14),phase 3 (15)

[0006] 2-pole levitation winding (12) (inner): phase 1 (16), phase 2(17), phase 3 (18).

[0007] In order to achieve a cost-favourable overall system thepossibility exists of reducing the number of winding systems and thus tosimplify the control electronics in addition to the mechanical layout.

[0008] As an example of a motor with a reduced number of windings amotor with common torque winding systems and levitation force windingsystems consisting of four concentrated coils should be explained whichwill be termed a bearingless single phase motor in the following.

[0009] In FIG. 3 the rotor and stator of a four-pole motor is shown inan external rotor embodiment. In this arrangement the rotor (35) ispreferably constructed in ring-shaped or bell-shaped design. With theaid of the four concentrated coils (31, 32, 33, 34) generation of atwo-pole and four-pole circulation distribution is possible, so that alevitation force in the x and y directions and torque can be producedindependently of one another. The determination of the individual phasecurrents takes place paying attention to the desired value setting forthe rotor position and speed of rotation, rotation, rotor angle ortorque after evaluation of the sensor signals for rotor position (x, y)and rotor angle of rotation (Φ).

[0010] In the preceding section in connection with the prior art thebearingless single phase motor and the multiphase rotary field motorswere described.

[0011] Both embodiments have partly considerable technical andeconomical disadvantages:

[0012] The bearingless single phase motor (FIG. 3) is only suited forapplications with low requirements with respect to the starting torque.These include, for example, drives for pumps, blowers, fans orventilators. In the simplest constructional form the bearingless singlephase motor requires only four individual coils. The starting weaknessof the single phase drive is brought about by the design. Whereas rotaryfield windings are used for the building up of the radial levitationforces the motor part only has one single phase alternating fieldwinding. There are thus critical angular positions of the rotor in whichthe starting torque is zero independently of the selected currentamplitude. Provision must therefore be made design-wise for the rotor tocome to rest only in positions which differ from the critical angularpositions. The moment of inertia of the drive thus enables the criticalpoints to be overcome in particular in the starting phase but also inthe steady-state operation. For many other drive tasks the startingtorque of this type of motor is too small.

[0013] The bearingless multiphase motor (FIG. 2) corresponding to theprior art does not have the disadvantage of low torque angularpositions. In contrast to the single phase variant it not only has itsown rotary field winding in the bearing part but also in the motor part.However the high coil numbers associated with the two rotary fieldwindings are disadvantageous here. Typical coil numbers of suchbearingless motors range as a rule between 36 (distributed three-phasewindings) and 12 (simple two-phase windings).

[0014] Bearingless motors, with small numbers of coils and without therestrictions of the field of application which result from the singlephase technology, would be desirable from the preceding considerationsand technically and economically extremely interesting for the drivemarket.

[0015] The inventive solution of this problem can be seen from theindependent patent claim. Preferred variants are defined by thedependent claims.

[0016] The substantially simplified construction of the magneticallyjournalled machine and also the simplified electrical control incomparison to the previously known solutions is of particular advantagein the inventive solution of the problem.

[0017] The invention set out in the patent claims satisfies theabove-named requirement to a high degree. A further decisive advantagelies, as a result of the asymmetrical winding construction, in thepronounced chording of the winding and the associated high damping ofharmonics in the field layout and circulation layout. Thischaracteristic enlarges the stability range of the position androtational speed controller, the accuracy of setting and also thequietness of running of the magnetically journalled drive system.

[0018] Embodiments of the invention will be explained in the followingwith reference to the drawings. There are shown in schematicrepresentation:

[0019]FIG. 1 a convention magnetically journalled system;

[0020]FIG. 2 a bearingless multiphase motor relating to the prior art;

[0021]FIG. 3 a bearingless single phase motor belonging to the priorart;

[0022]FIG. 4 a bearingless multiphase motor with an asymmetric statorsection and concentrated windings;

[0023]FIG. 5 angle-dependent variation of the phase currents for aconstant levitation force in the x direction;

[0024]FIG. 6 asymmetrical magnetomotive force (MMF) along the peripheryof the stator;

[0025]FIG. 7 angle-dependent variation of the phase currents for aconstant torque;

[0026]FIG. 8 asymmetrical field shape at the air gap periphery with thepermanent magnetic field faded out;

[0027]FIG. 9 flux density variation of the levitation force winding(four-pole) of a motor with symmetrical stator core (rotor: two pole);

[0028]FIG. 10 flux density variation of the torque winding (two pole) ofa motor with a symmetrical stator core (rotor: two pole);

[0029]FIG. 11 bearingless motor with distributed windings (individualcoils);

[0030]FIG. 12 bearingless motor with distributed windings (coil groups);

[0031]FIG. 13 bearingless motor with separate winding sets for thegeneration of levitation force and torque;

[0032]FIG. 14 bearingless motor windings around the stator yoke.

[0033] A possible embodiment of the invention will be described by wayof example in the following.

[0034]FIG. 4 shows a motor with five concentrated individual coils (41,42, 43, 44, 45). With this stator arrangement both the two pole and alsoa four pole rotary field, i.e. a two pole and four pole MMF, can beachieved at the same time by a corresponding supply of current to thecoils with superimposed current components. Thus, in cooperation withthe two poled MMF, torque can be achieved on a two pole rotor and radiallevitation forces can be achieved in cooperation with the four pole MMF.

[0035] The odd number of coils or limbs five, which is notwhole-numbered divisible by the two pole numbers four and two that areused, leads to an asymmetrical stator core and to asymmetrical MMF orfield distributions at the periphery of the stator or of the air gap.Accordingly, in dependence on the angular position of the rotor, on thedemand for levitation force and on the torque requirement the coilcurrents are to be determined such that the desired operating point isachieved.

[0036]FIG. 5 shows for this purpose, for a constant levitation force inthe x-direction independently of the rotor angle Φ of a two polepermanent magnet rotor with sinusoidal flux density distribution, theassociated levitation force coil current components, with I₁ designatingthe current through the coil 41, I₂ the current through the coil 42, I₃the current through the coil 43, I₄ the current through the coil 44 andI₅ the current through the coil 45. In FIG. 6 the flux density plot ofthe winding field in the air gap 61 (with the permanent magnet field ofthe rotor faded out) is shown for the initial angular position (Φ=0)schematically in comparison to an ideal sinusoidal four pole fluxdensity plot (62). The shape of a four pole asymmetrical field can berecognized from the flux density diagram. The asymmetry arises as aconsequence of the non-integer ratio of the number of limbs five to thewinding pole number four realized via the phase currents.

[0037]FIG. 7 shows in a manner matched to this, for a constant torque,likewise in dependence on the rotor angle Φ, the associated torque coilcurrent components and here I₁, again designates the current through thecoil 41, I₂ the current through the coil 42, I₃ the current through thecoil 43, I₄ the current through the coil 44 and I₅ the current throughthe coil 45. The current components shown in the two FIGS. 5 and 7 aresuperimposed in the five motor coils so that both the desired torque andalso the desired carrying force can be achieved.

[0038] The total MMF of the motor over the stator periphery arises fromthe electrical superposition of the currents and the geometricaldistribution of the coils. In FIG. 8 there is again shown the fluxdensity plot of the winding field in the air gap (81) (likewise with thepermanent magnetic field of the rotor faded out) for the initial angularposition of the rotor (Φ=0) in comparison to an ideal sinusoidal twopole flux density plot (82). The shape of a two pole asymmetrical fieldcan be recognized from the flux density diagram. The asymmetry arises asa consequence of the non-integer ratio of the limb number five to thewinding pole number two realized via the phase currents.

[0039] In the same manner bearingless rotary field motors can also bedesigned with for example six or seven concentrated individual coils. Athree-coil solution leads to a bearingless single phase motor withreduced mechanical cost and complexity.

[0040] In order to show the difference in the flux density shape to asymmetrically designed motor a four pole MMF and flux density plot (91)which arises with a motor with eight concentrated individual coils isshown in FIG. 9 in comparison to an ideal sinusoidal four pole MMF andflux density plot (92). For this motor configuration there are shown, inassociated manner in FIG. 10, a two pole MMF and flux density plot(101), again in comparison to a sinusoidal circulation and flux densityplot (102).

[0041] Instead of concentrated individual windings, distributed andoptionally chorded windings can be implemented into the stator. Here thetooth number or slot number of the stator is so selected for theabove-named reasons that it does not amount to any whole-numberedmultiple of the two winding pole numbers which are to be realized. Anexample to explain the principle construction is shown in FIG. 11. Wesee here five coils (111, 112, 113, 114 and 115) laid in slots with agreater coil width than in FIG. 4. The coils each surround two teeth andare thus no longer termed concentrated coils. A further variant is shownin FIG. 12 with an additional coil distribution, with two coils arrangedin adjacent slots forming a coil group electrically connected togetherin series or parallel.

[0042] Whereas the previously treated winding variants representintegrated variants which can simultaneously build up levitation forcesand torques, FIG. 13 shows an embodiment with separate winding sets forthe corresponding functions. The three phase two pole winding system 131a-131 b, 132 a-132 b and 133 a-133 b serves with a two pole permanentmagnet excitation for the generation of torque. With the aid of thethree-phased winding system 134 a-134 b, 135 a-135 b and 136 a-136 b afour pole flux density distribution can be produced which can be used togenerate levitation forces. The features of the invention can also berecognized in this variant.

[0043] A very simple and cost-favourable construction can be achieved bya mechanical arrangement such as is shown in FIG. 14. Here the coils(141, 142, 143, 144 and 145) surround the stator yoke (146) instead ofthe stator limb. If the stator yoke is assembled from segments simpleshaped coils can be inserted. The individual segments can be installedand positioned via a segment carrier, such as for example a plastic partmatched to the stator contour, via corresponding fastening means.

[0044] The variant shown in FIG. 14 offers the advantage that no parts(windings) projecting out of the stator surface are located in theregion of the stator teeth close to the air gap. Accordingly, parts ofpumps, blowers, fans, ventilators or others can be attached to the twosurfaces of the stator depending on the application.

[0045] In FIG. 15 an arrangement of this kind is shown. Here a part ofthe pump housing is directly located on the surface of the stator teeth.

1. Magnetically journalled electrical drive including a magneticallyjournalled electrical machine with windings inserted in the stator orrotor for the formation of torque and levitation force, a sensor systemfor determining the rotor positions and an analogue or digitalelectronic system for the control, regulation, monitoring and feeding ofthe magnetically journalled machine, with the electrical machine havingwindings which with appropriate current flow can produce magnetomotiveforces (MMFs) or magnetic fields with the pole numbers p₁ and p₂ with p₁and p₂ satisfying the condition p₁=p₂±2 and p₁ or p₂ representing thepole number of the rotor, in particular of a permanent magnet rotor,characterized in that the electrical machine is equipped for the p₁ andp₂ pole field production or MMF production in the stator with a number nof stator limbs (with concentrated windings), stator teeth (withdistributed windings) or stator slots, with the number n notsimultaneously representing a whole-numbered multiple of the pole numberp₁ and the pole number p₂.
 2. Electrical drive in accordance with claim1, characterized in that the stator segments between two axes throughthe stator middle point are distinguished in shape whereby the two axesstand orthogonal to one another.
 3. Electrical drive in accordance withone of the preceding claims, characterized in that the stator core isaxially symmetrical to at most one of two axes in the stator plane whichstand orthogonal to one another.
 4. Electrical drive in accordance withone of the preceding claims, characterized in that the winding segmentsbetween two axes through the stator middle point are distinguished inshape whereby the two axes stand orthogonal to one another. 5.Electrical drive in accordance with one of the preceding claims,characterized in that the winding arrangement is axially symmetrical toat most one of two axes in the stator plane standing orthogonal to oneanother.
 6. Electrical drive in accordance with one of the precedingclaims, characterized in that at least one of the two p₁ and p₂ pole MMFor winding field distributions has asymmetries over the stator or airgap periphery within an interval of one or more pole pitches, or iscomposed of non-periodic sections.
 7. Electrical drive in accordancewith one of the preceding claims, characterized in that the asymmetricalMMF distribution is in step-like form in accordance with the course oftwo superimposed p₁ and p₂ pole MMF functions, with at least one ofthese two functions having an angular step width of the stair stepswhich does not amount to any whole-numbered multiple of one or two polepitches.
 8. Electrical drive in accordance with one of the precedingclaims, characterized in that the stator winding is fully or partlydesigned as a distributed winding with coil groups which are composed ofmutually displaced individual coils connected to one another. 9.Electrical drive in accordance with one of the preceding claims,characterized in that the stator winding is fully or partly designed asa chorded winding that is to say with coil widths dissimilar to one ofthe pole widths present in the motor.
 10. Electrical drive in accordancewith one of the preceding claims, characterized in that the statorwinding is assembled from concentrated coils which each surround onestator limb.
 11. Electrical drive in accordance with one of thepreceding claims, characterized in that at least one limb or tooth whichis not wound is located between two stator limbs wound with concentratedcoils.
 12. Electrical drive in accordance with one of the precedingclaims, characterized in that separate winding sets are used for thelevitation force generation for the magnetic journaling and for thetorque generation for the motor drive, with one winding set only beingfed with currents for levitation force formation and the other windingset only being fed with currents for torque formation.
 13. Electricaldrive in accordance with one of the preceding claims, characterized inthat only one common winding set is used for the levitation forcegeneration for the magnetic journaling and for the torque generation forthe motor operation, with the coils of the winding set being operatedwith currents which contain both components for the generation ofsupporting forces and also components for the generation of the torque.14. Electrical drive in accordance with one of the preceding claims,characterized in that the winding set is so designed that with anappropriate current flow through the winding phases with superimposedcurrent components, a rotary field for the levitation force generationand a rotary field or alternating field for the torque formation can beproduced simultaneously.
 15. Electrical drive in accordance with one ofthe preceding claims, characterized in that the winding set for thelevitation force generation is designed as a rotary field winding. 16.Electrical drive in accordance with one of the preceding claims,characterized in that the winding set for the torque formation isdesigned as a rotary field winding.
 17. Electrical drive in accordancewith one of the preceding claims, characterized in that the winding setfor the torque formation is designed as an alternating field winding.18. Electrical drive in accordance with one of the preceding claims,characterized in that the coils enclose the stator yoke in a way thatthe coil axes essentially follow the circumferential direction or standnormal to rays through the centre point of the stator in order to obtainspace for mechanical or electrical assemblies, in particular for pump,compressor, fan or ventilator parts, in the region of the stator limbsand stator teeth close to the air gap.
 19. Electrical drive inaccordance with one of the preceding claims, characterized in that thestator yoke is mechanically subdivided into individual separate sectionsin order to enable a simple assembly of the coils.
 20. Electrical drivein accordance with one of the preceding claims, characterized in thatthe parts of the stator yoke are secured to an mounting carrier andindeed in such a way that as far as possible no considerable air gapsarise between the stator yoke parts.
 21. Electrical drive in accordancewith one of the preceding claims, characterized in that the motor has atwo or four pole permanent magnet rotor and three, five, six or sevenconcentrated stator coils, with the coils being fed with currents whichcontain both components for the generation of levitation forces and alsofor the generation of torque.
 22. Electrical drive in accordance withone of the preceding claims, characterized in that the motor has aplurality of phases which either consist of individual coils or of coilselectrically connected together in particular full-pitch coils, chordedcoils or distributed coils.
 23. Electrical drive in accordance with oneof the preceding claims, characterized in that the phases of the motorare star connected.
 24. Electrical drive in accordance with one of thepreceding claims, characterized in that with the motor phases starconnected, the star-point is not electrically connected.
 25. Electricaldrive in accordance with one of the preceding claims, characterized inthat the phases of the motor are connected in ring-like manner. 26.Electrical drive in accordance with one of the preceding claims,characterized in that the connected phase connections are driven viapower half bridges.
 27. Electrical drive in accordance with one of thepreceding claims, characterized in that the connected phase connectionsare driven via power full bridges.
 28. Electrical drive in accordancewith one of the preceding claims, characterized in that the currentdistributions of the motor phases distributed at the stator peripheryresult in a stator MMF of the kind which, viewed over the periphery,contains a p₁ pole fundamental wave and a p₂ pole fundamental wave. 29.Electrical drive in accordance with one of the preceding claims,characterized in that the drive has a p₁ or p₂ pole rotor with permanentmagnets.